This invention relates generally to semiconductor devices, and more particularly the invention relates to modulation-doped field effect transistors and fabrication processes.
In the continuing quest for higher speeds and transconductances in field effect transistors, bandgap engineering is being employed in device fabrication through use of heterojunctions in Column IV (e.g., SiGe) and III-V (e.g., AlGaAs) compound alloys. The Metal Oxide Semiconductor Field Effect Transistor (MOSFET) has evolved into the Modulation-Doped Field Effect Transistor (MODFET) by causing carriers to spill over from a high bandgap doped region into a low bandgap undoped region in a heterostructure. The carriers in the "modulation doped" channel have very high carrier mobilities because they do not suffer from the deleterious effects of ionized impurity scattering. This results in lower transit times, higher switching speeds, and increased drive currents and transconductances than in conventional MOSFETs.
The present invention is directed to an improved MODFET device which has hot carrier injection from a high bandgap source into a lower bandgap channel, with the carrier subjected to a built-in electric field along the channel region of the device.